1. Field of the Invention
This invention relates to devices for automatically adjusting a gap between a platen and a recording head in accordance with the thickness of a recording sheet. More particularly, the invention is directed to devices for adjusting the gap to an optimal value by judging the thickness of a loaded recording sheet.
2. Description of the Related Art
In a recording head, and particularly a wire dot type recording head that prints data by impinging wires onto a recording sheet through an ink ribbon, a wire impinging speed must be minimized in order to achieve high speed printing.
Since the wire dot type recording head is mechanically rigid and capable of copying data through a copying material, the recording head can be used with a wide variety of different types of sheets. This feature is problematic, however, because the distance between the recording head and the recording sheet varies by a greater degree than in printers of other types.
To overcome this problem, a printer using a wire dot type recording head usually has a mechanism for adjusting a relative gap length between the platen and the recording head. However, the user must be well skilled in selecting the optimal gap for each kind of sheet used, and the selecting operation is cumbersome.
The printer disclosed in Japanese Examined Patent Publication No. 4-14634 is intended to overcome the above noted problems. This printer has an encoder that generates pulse signals in association with the movement of the carriage from an initial position thereof, and a control section that processes a feedback pulse signal from the encoder. The first instance of a carriage-driving pulse motor losing synchronism is detected from a change in the number of pulses from the encoder due to an abutment of the recording head against a recording sheet, and the thickness of the recording sheet is calculated based on the amount of movement of the carriage up to the moment of abutment, so that the position of the carriage can be controlled based on the recording sheet thickness data.
Although the platen gap is adjusted as described above, unsatisfactory contact between the recording sheet and the platen causes the recording head and the ink ribbon to come in contact with the recording sheet, possibly causing problems such as breakage and staining of the recording head.